I received similar questions in the past weeks. As I am by far not an expert here, I have discussed this with my colleagues, who then came up with a proper response, which I have copied in below.

Tl:dr: It is not possible really, however it should also not make a difference in terms of SEO.

We have looked into options to offer you to build paths (e.g. */forum*) that point to the community, however managing those would be a real challenge for you and for us, with only a questionable benefit.

Contrary to popular belief, using either a subdomain or a subdirectory (or "path") setup for any given website module, such as a community forum, does not influence the main domain's rank (source: Matt Cutts, spokesman Google Search Quality Team). In the recent past, however, it was common practice to give a module its own subdomain (e.g. forum.domain.com) in order to separate it from the main website and prevent both rankings to possibly negatively influence each other. If a module was thought to be of positive influence and therefore an integral part of the website, a subdirectory subdomain (e.g. www.domain.com/forum) was supposed to be better suited. Nowadays, this has changed. Google explicitly states both to be evaluated "roughly equivalent" to each other because of the main domain name they share, regardless of where it is positioned within the URL. Each site will now be valued as a whole. Because the subdomain setup does provide great technical advantages for the inSided community platform to be integrated into any corporate website, this method is preferred over the other, since it has no negative consequences for a search engines rank.

To host the community platform in a recognizable, professional and reliable location, a new subdomain is created. This ensures that the new section is seamlessly integrated in the existing environment and gives inSided the possibility to offer its SaaS solution to the visitor. The subdomain will be pointed towards the inSided servers through a CNAME, a record in the DNS, so inSided can manage it.

Our experience with our many large clients is that subdomains work great in regard to search results. By using subdomains user-generated content, as well as company-generated content turn up in search results, with a visible distinction between these two types. These results are often really mutually beneficial. When working with 1 domain, the search engine will make choices based on changing and not always very transparent algorithms. The result in many cases will be that company website content or community content will be shown, instead of both.

I hope this will help, let me know if there is feedback on it or if something is unclear!